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Solvent Extraction in the Nuclear Fuel Cycle
Published in Reid A. Peterson, Engineering Separations Unit Operations for Nuclear Processing, 2019
Gabriel B. Hall, Susan E. Asmussen, Amanda J. Casella
Pulsed columns operate by applying mechanical energy to “pulse” liquid in the column up and down. As the fluid passes through plates in the column, it is forced into small droplets forming an emulsion. Flow in a pulsed column is countercurrent, like the flow in a packed column. The most common way to obtain the pulse in the system is to inject pressurized air into the system, which in turn pushes liquid into the column. Pulsing acts to push the solution up and down in the column, increasing the turbulence and kinetics of extraction. Perforated plates in the column act to generate droplets as the dispersed phase is pulsed through the plates. Plate design varies between countries and systems (Law and Todd 2008). While only slightly more complex than packed columns, the pulsing action reduces droplet size, thus increasing contact area and mass transfer rate making pulsed columns more efficient in achieving extraction.
Cooling during Fuel Removal and Processing
Published in Geoffrey F. Hewitt, John G. Collier, Introduction to Nuclear Power, 2018
Geoffrey F. Hewitt, John G. Collier
Solvent extraction is a process that allows separation of dissolved materials. Suppose we have two liquids that do not mix, such as oil and water. If we have a solution of two substances, A and B, in one of the liquids, and component B is soluble in the other liquid but component A is not, then we may solvent-ex-tract component B from the original mixed solution of A and B by essentially shaking up (”contacting") the solution with an immiscible liquid in which only B is soluble. By then removing component B from the resultant solution, we have achieved a separation of A and B. Various types of equipment are used in chemical engineering for this process, and it is beyond the scope of this book to go into them in detail. Probably the most commonly used devices in reprocessing plants use mechanical stirrers to mix the two liquids, followed by settling tanks that allow their separation, with each of the liquids containing the respective components. These are called mixer settlers. Alternatively, vertical pipes containing perforated metal plates may be used, with one fluid flowing up the pipe and the other flowing down it. To promote mixing of the fluids, such columns are subjected to pulses, and they are often referred to as pulsed columns. A typical pulsed column is shown in Figure 7.11. The first objective of solvent extraction in the reprocessing plant is the separation of the valuable uranium-plutonium mixture from the nitric acid solution, which also contains the fission products. This is done by contacting the nitric acid fuel solution with an organic solvent, typically tributyl phosphate (TBP) diluted with odorless kerosene (OK). In a typical extraction plant, all but about 0.1% of the uranium and plutonium in the fuel solution is removed into the TBP phase.
Hydrometallurgy — An Introductory Appraisal
Published in C. K. Gupta, T. K. Mukherjee, Hydrometallurgy in Extraction Processes, 2019
Solvent extraction or liquid-liquid extraction, a unit process defined earlier, is the process of transferring a substance in solution in one liquid into solution in a second liquid which is wholly or partially immiscible with the first. The process is, in essence, a simple technique and may be illustrated by an example in which X and Y are the two substances that are present in an aqueous phase (L1), as shown in Figure 21. This solution is then mixed with an organic liquid (L2) lighter than the aqueous phase (L1) and not miscible with it. The L1 and L2 are allowed to separate under gravity. The less dense L2 floats on L1, and if Y is more soluble in L2, it will contain Y but not X. By mixing L2 with another solution (L3), Y is stripped from L2 back into an aqueous phase, from which it can be recovered for use, and L2 can be recycled back to the beginning of the process. The given figure does not include a third process called scrubbing which is often adopted in the solvent extraction process for the removal of any coextracted impurities. The scrubbing step is incorporated after the extraction section and is accomplished by bringing the loaded solvent in contact with a fresh aqueous phase which takes away the impurities selectively. The essential step in solvent extraction is the intimate contact of two immiscible liquids for the purpose of mass transfer of constituents from one phase to the other followed by the separation of the two immiscible liquids. The device in which this is accomplished is known as a contactor. The simplest of the equipment is a packed column consisting of a vertical tube filled with metal or ceramic rings which break up the liquid phases (organic and aqueous) and force them into flowing through the column. The phases are separated solely by the difference in density of the two phases, the lighter organic phase flowing up through the column and the heavier aqueous phase down the column. The packed column does not provide a vigorous mixing. The flow rates are comparatively low. As a consequence of these, the packed columns need to be quite tall for good extraction. The increased efficiency and lower height required by pulsing the feed to the column has given way to another column design called pulsed columns. The principal alternative to the packed/pulsed columns for solvent extraction is a device known as the mixer-settler. In this equipment the aqueous and organic phases are mixed in a mixing chamber. The separation between the two phases takes place in a longer settling chamber which is connected with the mixing chamber. A typical solvent extraction circuit illustrating extraction and stripping stages in mixer-settler equipment systems is shown in Figure 22.
Regime Transition and Holdup in Pulsed Sieve-Plate and Pulsed Disc-and-Doughnut Columns: A Comparative Study
Published in Solvent Extraction and Ion Exchange, 2018
Nirvik Sen, Sourav Sarkar, K.K. Singh, S. Mukhopadhyay, K.T. Shenoy
Pulsed extraction columns are extensively used for carrying out liquid–liquid extraction. Pulsed columns are extraction columns in which the countercurrent flow of two liquids is driven by an external pulse. Various methods for providing an external pulse have been devised. Pulsing essentially sets the whole fluid inside the pulsed column in a to-and-fro motion. Such pulsations increase the level of turbulence, which in turn leads to generation of dispersion having high specific interfacial area, facilitating mass transfer. Pulsed columns are typically characterized by high separation efficiency and high throughput.[1] As separation efficiency of pulsed columns is more compared to conventional columnar extractors (such as sieve-plate columns, spray columns, etc.), the required height of pulsed columns is less for a given separation requirement. This makes these columns very attractive in chemical industries.[2,3] One of the mechanisms to provide the external pulse is by means of compressed air. This pneumatic mechanism avoids the presence of mechanical moving parts and eliminates the requirement of mechanical maintenance.